1. Field of the Invention
The present invention relates to inflatable packers having utility in downhole operations, particularly inflatable packers adapted for use in formation fluid sampling.
2. Background of the Related Art
Once an oil well has been drilled, it is often necessary for the operator to obtain downhole data, such as pressure measurements and downhole fluid samples for analysis. These tasks are commonly accomplished with downhole tools, such as modular wireline tools or drilling tools with evaluation capabilities, that employ probes for engaging the formation and establishing fluid communication to make the pressure measurements and acquire the fluid samples. Fluid is typically drawn into the downhole tool through an inlet in the probe. In some instances, such as for tight, low permeability, formations, sampling probes are often replaced by dual inflatable packer assemblies. Examples of such probe and packer systems are depicted, for example, in U.S. Pat. Nos. 4,860,581 and 4,936,139 assigned to Schlumberger, the entire contents of which are hereby incorporated by reference.
FIGS. 1A-1B schematically illustrate a typical configuration of dual packer elements 10 in their respective deflated and inflated conditions. The packer elements 10 are spaced apart along a downhole tool 12 conveyed by a wireline 14 in a borehole 18 penetrating a subsurface formation 20. Although a wireline tool is illustrated, other downhole tools conveyed by drill string, coiled tubing, etc. are also suited for such tasks. When inflated, the packer elements 10 cooperate to seal or isolate a section 16 of the borehole wall 18, thereby providing a flow area with which to induce fluid flow from the surrounding formation(s).
When inflating the packer elements (typically made of rubber), their ends often sustain large amounts of deformation and bending stresses, which may lead to circumferential tearing, and system failure. Additionally, since it is not uncommon for boreholes to exhibit high temperatures, particularly at great depths, the packer elements are often subjected to significant thermal stresses.
Attempts have been made to prevent packer failures. Accordingly, inflatable packer bodies or elements are often equipped with reinforcements in the form of metal cables or slats. While these reinforcements may be used to increase the life of the packer elements, the reinforcements may plastically deform and permit undesirable extrusion (as shown in FIGS. 1B-1C) under the high stresses imposed when the packer element is inflated and engages the wall 18 of a high temperature borehole. Additionally, the support members (i.e., the metal slats or cables) may have limited strength, and the flexible material of the packer element—typically rubber—may weaken with increasing temperature. The resulting deformation may be non-recoverable, thereby preventing the packer elements from retracting to within desirable diameters after sampling. In other words, the packers may fail to successfully return to the profile shown in FIG. 1A. Thus, when running these so-called “slat packers,” there is an increased risk of getting stuck in the borehole.
Despite the advances in packer technology, there remains a need for a packer with a long life under harsh wellbore conditions. It is desirable that such a packer limit or constrain the deformation that the packer undergoes during borehole operations so as to achieve a “milder” inflation profile (e.g., avoid the extruded profile of FIGS. 1B-1C) and thereby increase the life of the packer. Preferably, such a solution would be adaptable for use with known packer bodies or elements. It is further desirable that the packers retract to their original shape (e.g., as seen in FIG. 1A) so as to reduce the likelihood of a downhole tool getting stuck in a borehole. Preferably, such a solution would use ambient borehole fluid pressure to achieve the desired retraction, and balance the loads applied to each of the packers of the downhole tool.
A further issue that arises in dual packer assemblies relates to the axial separation distance between the packer elements. As this distance is increased, e.g., to increase the isolated area of the borehole wall, the risk of buckling at the mandrel that separates the packers typically increases. Accordingly, a need exists for a solution to the buckling risk in spaced dual packer assemblies.